Extruder for thermoplastic or non-cross-linked elastomeric material, a mixing zone defined by a feed screw

ABSTRACT

In an extruder for thermoplastic or non-cross-linked elastomeric material, a mixing zone defined by a feed screw for the material comprises at least one mixing ring on a cylindrical core portion of the screw, the mixing ring being apertured for the flow of material through the ring. The apertures are formed by a plurality of circumferentially distributed passages which are alternately oppositely inclined to the longitudinal axis of the screw.

United States Patent Upmeier 51 March 6, 1973 EXTRUDER FOR THERMOPLASTIC[56] References Cited OR NON-CROSS-LINKED UNITED STATES PATENTSELASTOMERIC MATERIAL, A MIXING N DEFINED B A FEED SCREW 51392;?gagldelllg ..259/ 191 V C ren Inventofl Hartmut p Tecklenburg, 3,300,8101/1967 Gregory Germany 3,564,651 2/1971 Covington ..425/200 [73]Assignee: ti l r u w. Bolscher Primary Examiner-Robert W. Jenkinsgenchlwestphaha Germany Att0rney-Fleit, Gipple & Jacobson [22] Filed:May 4, 1971 S A 57 AB TR CT [21] Appl. No.: 140,190 1 In an extruder forthermoplastic or non-cross-linked elastomeric material, a mixing zonedefined by a feed Foreign Applicatlon Priority Data screw for thematerial comprises at least one mixing May 15 1970 Germany P20 23 9105ring on a cylindricalcore portion of the screw, the mixing ring beingapertured for the flow of material through the ring. The apertures areformed by a plu- [22] (till. rality of circumferential), distributed p gwhich E g 194 97 are alternately oppositely inclined to the longitudinalaxis of the screw.

7 Claims, 4 Drawing Figures PATENTED R 6 I973 INVENTOR Hortmut UPMEIERhis ATTORNEYS EXTRUDER FOR THERMOPLASTIC OR NON- CROSS-LINKEDELASTOMERIC MATERIAL, A MIXING ZONE DEFINED BY A FEED SCREW Theinvention relates to the extrusion of thermoplastic and non-cross-linkedelastomeric material and more particularly to a feed screw to be used inextruders for such material.

In the extrusion of thermoplastic or non-cross-linked elastomericmaterial, high requirements are placed on the uniformity or homogeneityof the material and these requirements cannot be met with conventionalfeed screws without the provision of special homogenizing and mixingzones. Radial and axial homogeneity in the elongated stream of fedmaterial should exist with regard to the temperature of the material,its viscocity when molten (plasticity) as well as the distribution ofany additives, for example pigments, fillers and/or certain reagentssuch as propellants for foamed plastics or cross-linking chemicals.

To achieve homogeneity, various forms of feed screws exhibiting specialshearing and mixing zones have been developed. In a shearing zone havinga gap of definite width, the fed material is brought to a certainminimum viscocity whilst in a subsequent mixing zone or zones radial andaxial homogeneity are brought about, in some cases with a simultaneousreduction in temperature so as to avoid the disadvantages inherent inexcessively high temperatures. For example, it is known to insert in theextruder a so-called torpedo member which has a certain length and asmooth cylindrical surface. The mixing effect brought about by this hasproved inadequate. Mixing zones working on the principle of dividing theflow of the material have brought about better results. In oneembodiment a mixing rib extending in the same direction as the feedingthreads of the screw is provided with a plurality of radial slots orsuch a slotted mixing rib is set at an inclination opposite to that ofthe feeding threads. It is also known to use damming or baffle plateswhich are mounted on a smooth torpedo member and provided with amultiple of radial slots, usually in association with several other suchslotted plates.

Although the last mentioned construction effects much more thoroughmixing, it has been found that mixing in a radial direction is stillinadequate in cases where there is a considerable temperature differencebetween the feed screw and the wall of the cylinder in which the feedscrew rotates; consequently, the temperature at the center of the fedstream of material is several degrees higher than that in the marginalregions of the stream. This is particularly evident when using thermallyneutral feed screws, i.e. untempered feed screws, in which the mixingzone is machined rather deeply and therefore enhances a reduction in thetemperature of the stream of material.

The invention aims to provide an extruder in which the feed screwoperates on the principle of dividing the flow of the stream of materialand effects proper homogenization in a radial direction with respect tothe temperature of the material as well as its consistency.

According to the invention, there is provided an extruder forthermoplastic or non-cross-linked elastomeric material, comprising afeed screw rotatable in a cylinder, the feed screw having a core whichdefines an annual flow gap with the wall of the cylinder, wherein thefeed screw is provided with a mixing zone containing a mixing ring whichextends from the core of the screw to the cylinder wall and which isprovided with a plurality of apertures for the flow of materialtherethrough, the apertures being formed by oblique passages extendingalternately from near the core to near the cylinder wall, and viceversa.

With such a construction of mixing ring, and it is preferred to use twoor more such mixing rings at a spacing from one another, the moltenlayers near the center of the stream of material are interchanged withlayers from near the cylinder wall to bring about a better mixingeffect. Further, and this is particularly important for an untemperedfeed screw, the hot layers near the center of the stream are led towardsthe cylinder wall which is usually more intensively cooled and thecooler marginal layers are then returned from near the cylinder walltowards the feed screw to cool the latter, this bringing about aconsiderable improvement in the radial uniformity of the temperature ofthe fed stream of material.

As viewed in the direction of flow of the material, the mixing zone ispreferably preceded by a feeding and shearing zone, the length and widthof the aforementioned annular flow gap in the mixing zone giving avolume which is at least equal to the total volume of the gap in thefeeding and shearing zone so that the time for which the material is inthe mixing zone will be at least equal to the time for which it is inthe preceding zones. The or each mixing ring may be in the form of areplaceable disc on the feed screw. This simplifies production and makesit less expensive to replace worn parts.

The inlet ends of the passages in the or each mixing ring are preferablyprovided with inlet cavities which extend unilaterally in the directionof rotation of the feed screw. The outlet ends of the passages may havecorresponding outlet cavities which extend unilaterally opposite to thedirection of rotation of the feed screw. This facilitates feeding of thematerial with the least possible resistance along a substantiallyhelical course about the feed screw. Additional flow-dividing mixerelements may of course be disposed between the mixing rings to intensifythe mixing effect.

An example of the invention is illustrated in the accompanyingdiagrammatic drawing in which:

FIG. 1 is a side elevation of a feed screw;

FIG. 2 is a front elevation of a mixing ring as viewed in the directionof the arrows IlII in FIG. 1;

FIG. 3 is a section on the line III-III in FIG. 2, and

FIG. 4 is a projection through a row of passages in the mixing ringtaken on the line IV-IV in FIG. 3.

The feed screw is provided with a feeding section A, v

a shearing and homogenizing section B and a mixing section C. Theshearing section B is illustrated in simplified form as a multi-ribbedring which is more fully described in our prior British PatentApplication No. 26218/71 (German Application P 20 19 522.6).

The feed screw of FIG. 1 is designed to rotate in a cylinder of anextruder, the inside diameter of the cylinder wall being indicated bythe arrows 4D. The core of the feed screw is much larger in the feedingsection A than in the mixing section C and consequently an annular flowgap defined between the cylinder wall and the core of the feed screw iswider in the mixing section or zone C, the width preferably being from0.15 to 0.20 times the internal cylinder diameter. In the illustratedembodiment, the mixing section C is provided with three mixing ribs 1which, for production reasons and to facilitate replacement after wear,are in the form of separate rings placed over the core of the feedscrew.

Machined into each ring 1 there are a plurality of alternativelyoppositely intersecting passages 2, 3 which are juxtaposed in acircumferential direction. At least the upstream sides of the passagesare provided with unilateral inlet cavities 4, 5 but correspondingoutlet cavities 6, 7 are preferably also provided on the downstreamsides of the passages. When forming these cavities, the intendeddirection of rotation of the feed screw (indicated by the arrow 8)should be taken into account because it is to be instrumental in causingthe overall flow 9 of molten material to follow a substantially helicalcourse about the screw core. The material readily enters the passagesthrough the inlet cavities 4, 5 and leaves them through the cavities 6,7 whilst encountering the least possible resistance to flow.

By means of the repeated interchange of material from near the core tonear the cylinder wall and vice versa, the fed material is intensivelyhomogenized and, as is particularly important for untempered feedscrews, the hot material near the core of the screw approaches theusually more intensively cooled cylinder wall whilst the relatively coolmarginal layers of material approach the core and reduce the temperaturethereof, thus improving the overall radial uniformity in the temperatureof the stream of material.

To avoid excessive shearing action on the material located in the mixingsection C, the annular gap between the cylinder wall and the feed screwcore in this section should be quite deep whilst the length of themixing section is preferably chosen so that the volume of extrudablematerial is located in the mixing section is at least equal to the totalvolume in the feeding and shearing sections, whereby the time for whichthe material is located in the mixing section will likewise be at leastequal to the time it stays in the preceding sections.

The mixing rings 1 can be fairly easily machined on copying millingmachines or on normal milling machines with indexing heads.

It is within the scope of the invention as defined in the appendedclaims to provide between the individual intersecting passagesadditional holes such as axial holes and/or radial slots, although theradial mixing effect would then be somewhat worse. Similarly, the mixingsection C could be interposed intermediate the length of the feedingsection A but those screwthreads of the feeding section disposeddownstream of the mixing section would then detrimentally influence thehomogeneity of the molten material. Further, in order to intensify themixing effect, radial kneading cams or pins or slotted discs or otherflow-dividing mixing elements may be provided between the mixing rings1.

I claim:

1. An extruder for thermoplastic or non-cross-linked elastomericmaterial, comprising a feed screw rotatable in a cylinder, the feedscrew having a core which defines an annular flow gap with the wall ofthe cylinder, wherein the feed screw is provided with a mixing zonecontaining a mixing ring which extends from the core of the screw to thecylinder wall and which is provided with a plurality of apertures forthe flow of material therethrough, the apertures being formed by obliquepassages extending alternately from near the core to near the cylinderwall, and vice versa.

2. An extruder according to claim 1 comprising two or more such mixingrings at a spacing from one another.

3. An extruder according to claim 1, wherein the mixing zone ispreceded, as viewed in the direction of flow of the material, by afeeding and shearing zone, the length and width of said annular flow gapin the mixing zone giving a volume which is at least equal to the totalvolume of the gap in the feeding and shearing zone.

4. An extruder according to claim 1, wherein the or each mixing ring isin the form of a replaceable disc on the feed screw.

5. An extruder according to claim 1, wherein the

1. An extruder for thermoplastic or non-cross-linked elastomericmaterial, comprising a feed screw rotatable in a cylinder, the feedscrew having a core which defines an annular flow gap with the wall ofthe cylinder, wherein the feed screw is provided with a mixing zonecontaining a mixing ring which extends from the core of the screw to thecylinder wall and which is provided with a plurality of apertures forthe flow of material therethrough, the apertures being formed by obliquepassages extending alternately from near the core to near the cylinderwall, and vice versa.
 1. An extruder for thermoplastic ornon-cross-linked elastomeric material, comprising a feed screw rotatablein a cylinder, the feed screw having a core which defines an annularflow gap with the wall of the cylinder, wherein the feed screw isprovided with a mixing zone containing a mixing ring which extends fromthe core of the screw to the cylinder wall and which is provided with aplurality of apertures for the flow of material therethrough, theapertures being formed by oblique passages extending alternately fromnear the core to near the cylinder wall, and vice versa.
 2. An extruderaccording to claim 1 comprising two or more such mixing rings at aspacing from one another.
 3. An extruder according to claim 1, whereinthe mixing zone is preceded, as viewed in the direction of flow of thematerial, by a feeding and shearing zone, the length and width of saidannular flow gap in the mixing zone giving a volume which is at leastequal to the total volume of the gap in the feeding and shearing zone.4. An extruder according to claim 1, wherein the or each mixing ring isin the form of a replaceable disc on the feed screw.
 5. An extruderaccordiNg to claim 1, wherein the inlet ends of the passages areprovided with inlet cavities which extend unilaterally in the directionof rotation of the feed screw.